This invention relates in general to aircraft navigation and, more specifically, to a navigation system using sequences of linear feature navigation updates to correct a basic inertial navigation system.
In the earliest days of aviation, cross-country navigation consisted merely of the pilot flying at low altitudes and low speeds while observing the countryside. Sometimes, pilots would simply use a roadmap and follow roads or railways between towns or airports. Of course, this method was poorly suited for night or bad weather flying, and could no longer be used as aircraft became faster and flew at higher altitudes. In those days, unmanned missiles were incapable of inflight guidance or navigation updating, flying a ballistic trajectory from launch.
Later, a variety of radio beacons and the like were developed to aid commercial airline and private aircraft navigation. Still, no truly accurate method of navigating unmanned vehicles was available and long-distance military airplanes, e.g., bombers, had to rely on stellar navigation.
The development of inertial navigation using precision accelerometers and gyroscopes greatly increased the accuracy of navigating aircraft, manned and unmanned, over long distances. While these systems are generally sufficiently accurate for manned aircraft, where the pilot could use visual aids to correct for small errors near the target, destination airport, etc., they have sufficient gyro drift and other errors to cause significant target error for unmanned vehicles, such as cruise missiles, flying long distances.
The next advance in aircraft navigation was terrain following navigation, useful for low flying aircraft, missiles, etc.. Here, an inertial navigation system is used to keep the air vehicle nearly on the intended flight path. A terrain elevation map, which had been earlier prepared, of selected areas along the flight path is stored in computer memory. As the vehicle approaches the mapped area, a radar altimeter senses changes in ground contour and compares them to the stored terrain map. The computer matches the actual sensed terrain with one path across the mapped area, determining the actual flight path and the degree of error. The inertial navigation system is thus updated to return the vehicle to the intended flight path. A similar method used digital scene matching. This technique compares a sensed scene, taken by a TV sensor, with a reference scene.
While highly accurate, these systems have a number of problems and deficiencies. Preparing a digital scene matching map of a selected area is a very tedious and expensive undertaking. The map must be prepared from very high quality stereoscopic photographs taken over areas generally controlled by a potential enemy. The digital map uses a great deal of computer storage capability. In some cases, changes in terrain, e.g. snow cover, trees in or out of leaf, etc. may confuse the vehicle sensors. Good reference scenes, i.e. areas having sufficiently rough terrain, may not be available along the desired route. In the case of a warplane or missile, the areas with "good" terrain features near a likely target may be obvious to both sides. If these regions are isolated, an attacking plane or missile must be "funnelled" through them to provide high navigation accuracy. The enemy may be able to concentrate his defenses in these regions to thwart an attack.
Thus, there is a continuing need for improved aircraft navigation systems, particularly for unmanned missiles which must travel long distances at low altitudes to precisely impact a target.